Diversifying Variable Renewable Energy Sources to Reduce Utility Reserve Requirements

Sources of renewable energy, including solar, offshore wind, and ocean wave technologies, offer significant advantages including no fuel costs and no emissions from generation. However, the renewable and nondispatchable nature of these technologies severely impacts grid reserve requirements. Like many areas in the U.S., the Pacific Northwest is rapidly expanding its wind power resources. An additional 5000 MW of offshore wind power is expected to come online in this area in the next five years. This trend in renewable energy resource development presents significant problems for system operators. The variability of wind resources can create a need for greater ramp-up rates, interhour variability, and scheduling errors in conventional power plants. These factors combine to increase the amount of energy generation capacity the system operators must hold in reserve to prevent rolling blackouts and energy shortages. Halamay and colleagues (2011) analyzed the interaction of variations in utility load, wind power generation, solar power generation, and ocean wave power generation. Their research suggests that a diversified portfolio of energy resources can reduce the effects of variability and decrease utility reserve requirements.  —Meredith Reisfield
Halamay, D.A., Brekken, T. K. A., Simmons, A., McArthur, S, 2011. Reserve requirement impacts of large-scale integration of wind, solar and ocean wave power generation. IEEE Transaction on Sustainable Energy 2, 321–328.

Halamay and his colleagues analyzed the effects of offshore wind, solar and ocean wave renewable energy sources on reserve requirements for the Pacific Northwest. The output of each of these renewable power sources varies over time. While the variation is typically small, the output of a large plant can occasionally go from full output to low production or vice versa over the course of several hours. System operators also have limited control over renewable power generation, so in this analysis the researchers chose to subtract the contribution of renewable energy sources from the total load. The researchers hypothesized that a diversified renewable energy portfolio would enable a greater penetration rate than just one predominant renewable energy source. Penetration is the ratio of the peak load within the year to the peak generation within the year. In each of these scenarios is greater than or equal to penetration by solar and wave energy.
The researchers calculated the energy reserve requirements for six scenarios; no renewable energy; 15% wind power penetration; 10% wind and 5% solar penetration; 10% wind and 5% wave penetration; 10% wind, 2.5% solar, and 2.5% wave penetration; and 5% wind, 5% solar, and 5% wave penetration. The second scenario, 15% wind generation, most closely reflects the current energy portfolio in the area studies, which has 14% wind penetration. The researchers studied the area within the Bonneville Power Administration (BPA) Balancing Authority Area (BAA). Wind generation and load data were freely available from the BPA. Wind power data were collected from approximately 1600 MW of wind under the BPA BAA, which was scaled as necessary to model the desired penetration rate. Irradiance data were gathered from 10 different locations in the Pacific Northwest to calculate potential for solar power generation, with the assumption that each location hosted 50% photovoltaic and 50% concentrating solar sources. The data were combined, weighting each site equally, and scaled to model the desired power generated for each particular scenario. Buoys measuring wave height at three locations were used to calculate theoretical ocean wave power outputs. The data were also combined and scaled as necessary. All data and analysis focused on the 2008 calendar year and used 10-minute sample times.
The researchers also used three different time scales to describe power reserve requirements. The first, regulation, examined the difference in small changes in power that can be readily met through Automatic Generation Control (AGC) via spinning reserves. The following time scale, defined as the difference between hourly power generation and 10-minute average power load describes larger changes in power demand and supply. The imbalance time scale describes the accuracy of forecasted power generation by comparing hourly forecasted power generation with hourly average power generation. Imbalance components of reserve requirements have grown significantly with increased use of wind power and are predicted to continue to grow rapidly. The 2008 load was forecasted using historical data from 2007 as a baseline, with additional correction terms added to account for load-growth from one year to the next and smooth transitions between monthly averages to prevent discontinuities. The scenarios with diversified renewable energy sources showed improvement over use of wind alone.
Halamay and his colleagues demonstrated the adverse affect of wind power on reserve requirements. These results suggest the need for an integrated approach to develop renewable energy sources. This analysis did not include tidal energy conversion, harvesting energy from tidal flows to generate power, despite the strong tidal resource in populous areas such as Puget Sound. 

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